30. Cytological investigation on an isotetrasomic in rice Zhukuan Cheng, Bengxiu Yu and Minghon Gu
Agricultural College, Yangzhou University, Yangzhou, 225009 China     Primary trisomics of rice have been the subject of several studies. With the aid of primary trisomics, a number of marker genes have been located to rice chromosomes.  
    The corresponding relationships of chromosomes and linkage groups has been established using the primary trisomics. Secondary trisomics, in which the extra chromosome is an isochromosome, have been reported in Datura, barley, maize and tomato. Only a few reports on isotetrasomics are available.
    Zhongxian 3037 is a midseason variety of indica rice developed from a cross between IR24 and BG90-2. An aneuploid plant with 26 chromosomes was found in the progeny of triple 10 of this variety. Because the two extra chromosomes are isochromosomes, we designate it as an isotetrasomic.
    Cytological investigation revealed four modes of chromosome pairing in PMCs at diakinesis and metaphase I, i.e., 13II, 11II+1IV, 11II+11II+1I, and 12II+2I. Since the extra chromosomes were often attached with nucleoli, the aneuploid was obviously a
tetrasomic related to nucleolar chromosomes. Zhongxian 3037 has two nucleolar chromosomes, i.e. chromosome 9 and 10, respectively, according to the chromosome numbering system agreed upon during the 2nd International Rice Genetics Symposium in 1990 (Khush 1990). Chromosome 9 is slightly longer than chromosome 10. The two extra chromosomes of the isotetrasomic revealed different pairing configurations at pachytene. The internal pairing often occurred between the two arms of each isochromosome and it was often attached to the nucleolus (Fig. 1 A). The two extra chromosomes could also pair with each other, forming a bar or a ring (Fig. 1B, 1C). Further study on the pairing behavior indicated that one of the internally paired isochromosomes was often attached to the short arm of chromosome 10 or was directly connected with centromere of this chromosome (Fig. 1D, 1E), while the second internally paired isochromosome was attached to the nucleolus. Other configuration could often be observed as the isochromosomes paired with the short arm of chromosome 10 forming a "Y" shape near the nucleolus (Fig. IF). All the synaptic configuration and the similarity in length and chromomere distribution indicate that the aneuploid is an isotetrasomic for the short arm of chromosome 10. The isotetrasomic is also different from triple 10 in morphology. It is shorter in plant height and dark green in color. The spikelet fertility of the isotetrasomic is much lower (23.5%) than that of triple 10 (about 57.5%).

References

Cheng, Z. K., M. H. Gu, 1994. Karyotype analysis for pachytene chromosome of indica,japonica rice and their
    hybrid. Chinese Journal of Genetics 21(3): 181-190.
Khush, G. S., 1990. Report of meeting to discuss chromosome numbering system in rice. RGN 7: 12-14.